1. Field of the Invention
This invention relates to multiple stage gear speed change systems and, more particularly, to a speed change system, which can obtain a wide speed change ratio range and has reduced necessary gear sets so that it is suited for mounting in vehicles.
2. Prior Art
FIG. 20(A) accompanying the specification schematically shows a well-known gear speed change system. This speed change system comprises an input shaft, an output shaft disposed coaxially therewith, and a counter shaft disposed parallel to the input and output shafts. The input and counter shafts are coupled by a gear pair or set A to each other at all times. More specifically, a gear rotatable in unison with the input shaft and a gear rotatable in unison with the counter shaft are in mesh with each other and form the gear set A coupling the input and counter shafts at all times. The counter and output shafts are coupled by a plurality of gear sets to each other. These gear sets are each formed by at least two gears, at least one of which is rotatable relative to the output or counter shaft. A clutch is disposed adjacent the gear sets. The clutch can render one of the plural gear sets unable to rotate relative to the output and counter shafts while rendering the other gear sets able to rotate relative to the output or counter shaft. The plurality of gear sets individually have different speed reduction ratios. In the Figure, the solid frame between shafts indicates a gear set which is coupled at all times, and dashed frames indicate gear sets which are coupled and de-coupled by the clutch.
In the example of system shown in FIG. 20(A), "reverse" is set up when the gear set B is coupled, "first forward" is set up when the gear set C is coupled, "second forward" is set up when the gear set D is coupled, "third forward" is set up when the gear set E is coupled, "fourth forward" is set up when the input and output shafts are coupled directly, and "fifth forward" is set up when the gear set F is coupled. The system has a total of six gear sets.
In this type of speed change system, if it is desired to obtain a wide speed change ratio range, it is necessary to greatly change the gear diameters of the first and fifth forward gear sets C and F, thus posing limitation on the practically realizable speed change ratio range.
Japanese Laid-Open Utility Model Publications Nos. 1-169644 and 2-146335 disclose techniques for solving the above problem. In the disclosed techniques, two counter shafts are used as shown in FIG. 20(B) accompanying the specification. Actually, the two counter shafts are disposed coaxially. Of the two counter shafts, one (i.e., a second counter shaft in the illustrated case) is for low speed, and the other (which is a first counter shaft in this case) is for high speed. The input shaft is coupled at all times to the second and first counter shafts by gear sets D and A, respectively. The second counter shaft for low speed is rotated at a lower speed than the first counter shaft for high speed. The output and second counter shafts are coupled by low speed gear sets E to G, and the output and first counter shafts are coupled by high speed gear sets B and C. A clutch can selectively cause the gear sets B and C and those E to G to transmit torque, and it is possible to realize five different speed change ratios each by selectively coupling each of the five gear sets B to G for torque transmission. In addition, a further speed change ratio is obtainable by coupling the input and output shafts directly. That is, a total of six speed change ratios are available for selection. Thus, this speed change system, like that shown in FIG. 20(A), is a reverse 1, forward 5 speed change ratio system. In this case, the rotational speeds of the two counter shafts can be changed greatly to provide a great span between the first and fifth forward speed change ratios, that is, it is possible to obtain a wide speed change ratio range.
While the structure shown in FIG. 20(B), using two counter shafts, can provide a wide speed change ratio range, a gear set is necessary between the input shaft and each of the two counter shafts, that is, one more gear set is necessary than those in the structure shown in FIG. 20(A). With the structure shown in FIG. 20(A), on the other hand, a sufficiently wide speed change ratio range can not be obtained.